Monitoring the HIV epidemic to understand rates and patterns of growth, as well as targeting intervention efforts to populations exhibiting high rates f HIV transmission, are wholly dependent on determining the frequency of new infections using an assay that discriminates recent from long-term HIV infection. However, very few HIV assays have been developed specifically to distinguish incidence from prevalence. An added barrier has been the lack of an incidence assay for field work and resource-poor settings. Most HIV serologic assays are aimed at diagnostic use, while RNA assays have been used largely to determine viral load for clinical management purposes. Furthermore, HIV incidence testing currently requires access to well-equipped centralized laboratories capable of running the few sophisticated assays available for this purpose; these have been ELISAs requiring microplate handling and reading instrumentation, including the BED ELISA and the Vironostika detuned ELISA. Dependence on a central laboratory also implies the requirement for a system to transport serum specimens from where they have been collected to the laboratory, a separate and acute logistical challenge. In this project, we propose to transform HIV serological incidence testing from ELISA methodology based on avidity and titer and requiring a laboratory with sophisticated infrastructure, to a field procedure using a simple, stable, and reliable rapid test.In Phase I, we have developed prototypes of a new, highly sensitive rapid test based on novel colorimetric detection technology, which allows the analysis of antibody titer and avidity in less than 15 minutes within a single cassette. The rapid assay is based on synthetic peptides derived from a recently developed multisubtype gp41 HIV peptide antigen (IDR-M) which is the basis for the currently state-of- the-art limiting antigen (LAg) ELISA for incidence testing. Evaluated with a CDC incidence/prevalence panel, the prototype rapid assay yielded > 98% correlation with the LAg ELISA. In Phase II, we will evaluate the accuracy of the new rapid test for classification of recent vs. long-term HIV infections using well-characterized serum/plasma panels developed specifically for this purpose. These will include a panel representing recently HIV-infected individuals (HIV RNA positive/anti-HIV negative at recruitment) collected over a two year period as a specific aim in Phase I. Field evaluations of the rapid test will be carried ot in collaboration with public health agencies at global sites in low-resource regions to determine its performance characteristics and operability. The new rapid HIV incidence test will serve a major public health need by bringing testing to identify new HIV infections to the field, where the subject is located, enabling more efficient monitoring of the HIV epidemic at a local level and more effective intervention tactics.